A wind turbine obtains power by converting the force of the wind into torque acting on the drive train, i.e., on the rotor blades and thus on the main shaft and thereby typically on an electrical generator rotated by the main shaft directly or through a gearbox. The amount of power which the wind turbine receives and which therefore potentially can be transferred to the drive train depends on several conditions including the wind speed and the density of the air, i.e., the site conditions.
Even though a desire to increase productivity requires conversion of the highest possible amount of wind energy to electrical energy, the structural limitations of the wind turbine, i.e., the design loads, define safety limits for the allowed load on the wind turbine. In practice, the wind load depends on various weather conditions including the average wind speed, wind peaks, the density of the air, the turbulence, wind shear, and shift of wind, and the impact of the wind load on the wind turbine and thereby the load on the wind turbine can be adjusted for a current wind condition by changing various settings on the wind turbine.
Even though the loading of a wind turbine is determined by a number of weather conditions and settings on the wind turbine, the wind turbines of today are typically controlled in accordance with a relatively simple and reliable control strategy according to which the turbine is shut down completely at wind speeds above a certain safety value.
Though representing a potentially very safe way of operating a wind turbine, the complete shut down represents drawbacks, for example, due to the fact that any major change in power production may influence the supplied power grid in a negative way.